Science Buddies: "Ask an Expert"

Does the weight of an object affect how fast it will move in salt vs fresh water? Also, how about the surface texture of the object (smooth vs bumpy) and more importantly why?
Thanks for you help

Hi,

You need to look into viscosity as a phenomenon. Here are Science Buddies projects on the subject:

https://www.sciencebuddies.org/science- ... p055.shtml

https://www.sciencebuddies.org/science- ... p019.shtml

Here is a study of the effect of surface roughness on drag in ships, with explanations as to cause:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD ... tTRDoc.pdf

http://www.onepetro.org/mslib/servlet/o ... MS&soc=OTC

http://en.wikipedia.org/wiki/Drag_(physics)

I hope this helps answer your question.

Best regards,

Barrett L. Tomlinson

I looked at them, however they are a little too technical. This is for my son who is autistic and I have
to go through and read everything to him and explain it in simple terms. I'm not sure if I understood what I read.
I have further narrowed his project down to just explaining why the surface texture of the 3 balls produced the rate of motion that they did in salt water vs tap water.
I really appreciate any further help you can give.

Hi,

In highly oversimplified terms, balls fall faster in tap water than salt water because in salt water the net force pulling them down is less than in tap water. There is a simple way to demonstrate this. Hang the ball to the bottom of a spring scale so it measures the weight of the ball. Dip the ball into tap water and record the apparent weight, then do the same thing in salt water. The apparent weight of the ball in salt water will be lower than the apparent weight in tap water. The ball falls faster when there is more force pulling it down.

As to the effect of surface roughness on the rate of falling, a smooth surface ball falls faster than a rough surface ball. A simple analogy experiment can be done. Take a piece of wood and slide a pience of paper over it - it slides pretty easily. Try sliding sandpaper over the wood - it is harder to make it slide. It is harder to make a coarse grade of sandpaper slide than a fine grade of samdpaper. So too with the balls in water. A smooth ball slides through liquid with relative ease. A rough surface ball sticks to the liquid more, so slides through the liquid slower.

These are extremely oversimplified explanations, but might make sense to your son.

Good luck to you both!

Barrett L. Tomlinson